Currently, getting a damaged metallic or composite component ready for a bonded repair requires the technician to gain knowledge of the thermal characteristics of the repair area. Structural stiffeners, frames, thick doublers, and thin skins all accentuate the thermal variations during the cure process. Adhesive materials and composite material resin systems require heat to be cured. The resin chemistry requires the heated area to be within a very narrow temperature range for the resins to cure properly. The presence of heat sinks can cause the actual temperature variation to fall outside of the specified range. Any un-cured, undercured, or overcured resins will cause a failure in the repair. By understanding the thermal anomalies of the structure a cure plan can be developed that includes selection of the heating source and insulation methods needed to equalize the cure temperatures within the specified range prior to curing the part thus ensuring a well fully cured component.
As part of the thermal survey a vacuum leak check is often performed. Vacuum leaks around fasteners, between attached structures, and through holes in the structure can filter air and other gases through the adhesive/resin materials being cured thus creating porous bondlines and resins. By understanding and sealing the leak paths in the repair area, porosity will be minimized and the repair will pass any subsequent inspections.
Existing solutions for bonding a repair onto a structure includes a thermal survey and a vacuum leak test prior to the actual repair. To perform a survey the repair in reality needs to be performed two times. The first “repair” is the survey where a full-size doubler is constructed out of actual repair material using one of the templates created that identifies each ply boundary. A 60-ply repair can take hours cutting and orienting repair plies into a suitable doubler. This new doubler is then placed onto the prepared repair area using a separator ply (so the survey doubler does not bond to the structure). Numerous thermocouples are placed in the curable and adjacent areas to monitor temperatures during the cure. A vacuum bag and heat blanket stack-up is then prepared and placed over the doubler. The survey doubler is then cured using the actual equipment and process spelled out in the repair procedure. The thermocouples are monitored and insulation is added to low temperature regions of the repair and insulation is taken away from hotter sections of the repair. In some cases supplemental heat is applied to the backside of the repair or to the perimeter to act as a thermal dam thus reducing the cure temperature variation. In addition the thermal monitoring, vacuum gauges are monitored to ensure adequate vacuum is available to apply the required cure pressure to the repair. Low vacuum would indicate a vacuum leak and an opportunity for air or other gases to permeate the resin or bondline thus reducing the repair performance to the point of a failed repair. After the survey is complete and a cure plan is ready for the actual repair, the second set of repair materials can now be used to complete the actual repair.
Existing solutions double the time necessary to perform a bonded repair, require two complete sets of repair consumables including very expensive repair materials, and add another heat cycle to the parent structure. Parent composite structures are often limited to five heat cycles during the life of the structure before replacement is necessary due to heat degradation of the polymers in the composite material.
A time-efficient thermal and vacuum assessment method for determining the thermal variations and vacuum leak paths in a damaged structure prior to installing a bonded repair to ensure uniform heating and minimal porosity of the repair is needed.